i did an emerge of this last night and it did not fully install all the kernel modules. i had to perform "sudo depmod -a" to complete the install.
18 Sep 2012; Lars Wendler <polynomial-c@gentoo.org> -virtualbox-guest-additions-4.1.12.ebuild, -virtualbox-guest-additions-4.1.18.ebuild: Removed old versions. You should perhaps sync your portage tree again. Try with a newer version and reopen this bug report if that fails too.